A high-pressure tank in which gas such as hydrogen gas is stored under high pressure is mounted on, for example, a fuel cell vehicle. A ferrule of the high-pressure tank is disposed at an opening of a tank body, and the opening is provided at an end portion of the high-pressure tank (see, e.g., Patent Document 1). Moreover, a valve body having a piping element such as a valve, incorporated therein, is screwed in and connected to the opening of this type of ferrule. To secure sealing capability of the high-pressure tank, seal members are disposed between the ferrule and the tank body or between the ferrule and the valve body (see, e.g., Patent Documents 1 to 3). For example, in Patent Document 1, as a seal system for improving this type of sealing capability, a double seal structure is adopted in which axial sealing is provided by two O-rings.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-161590 (FIG. 3)    [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-279000 (FIGS. 2 through 4)    [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-349796 (FIG. 2)